This invention relates to a thermally stabilized IMPATT oscillator and, more particularly, relates to an IMPATT oscillator thermally stabilized for pulsed operation which achieves constant power and frequency output over a wide ambient temperature range.
Semiconductor devices produce heat as a byproduct of normal operation. This effect is especially significant with power semiconductor devices, such as IMPATT diodes which are used as sources of high frequency oscillation. With these devices the heat which is generated is usually an unwanted byproduct. The temperature has an impact on device operation so that output power and frequency may be affected by uncontrolled temperature variations. See H. M. Olson, et al., "Temperature Transients in IMPATT Diodes", IEEE Trans. on Elect. Dev., ED-23, pp. 494-503 (1976); W. E. Schroeder, et al., "The Effect of Temperature on the Operation of an IMPATT Diode", Proc. IEEE (Lett.), vol. 59, pp. 1242-1244, August 1971; and J. R. Grierson, "Theoretical Calculations on the Effect of Temperature on the Operation of IMPATT Diode", Electron. Lett., vol. 8, pp. 258-260, May 1972. If the heat is of sufficient magnitude and is present for sufficient lengths of time, device operation may be permanently impaired due to impurity migration or the IMPATT package may rupture. Thus, for continuous wave operation, one normally employs low thermal resistance, high thermal mass heat sinks to keep the junction temperature less than a critical operating temperature, e.g., 200.degree. C. If operating conditions, such as ambient temperature remain generally constant, then an equilibrium junction temperature will be reached; however, ambient temperature will likely vary over a range thereby producing changes in junction temperature and associated changes in operating characteristics.
For pulsed operation of the type useful in radar applications, especially pulsed operation of low duty cycle (generally less than 30% and often only several percent) heat dissipation is not the chief concern. In fact, for a given amount of heat that may be tolerated by the device and its package, higher peak powers are possible since the heat generated in short bursts of power is averaged out over time. Rather, it is desired to operate the IMPATT oscillator at a constant temperature in order to ensure uniform operating characteristics. Thus, with IMPATT diodes operating in the pulsed mode it is desirable to obtain thermal stability. This stability could be obtained by applying a variable amount of heat by an external heater to always keep the diode junction at a specified temperature. However, such a technique requires the heating of a large thermal mass so that high thermal time constants and long stabilization times are inherent. Also, it is bulky, heavy and wastes energy.
It is therefore an object of the present invention to provide a thermally stabilized IMPATT oscillator for operation in the pulsed mode wherein the diode junction has a bias voltage applied during non-oscillating subperiods to draw sufficient current across the junction so that on a time averaged basis a desired junction temperature is maintained over a wide ambient temperature range.
It is an additional object of the present invention to provide an IMPATT oscillator having a stabilized temperature characteristic which utilizes a platform for mounting an IMPATT diode which has a preselected thermal impedence.